This proposal deals with the development of a radiation detector that will make possible the direct measurement of the structure of charged particle tracks in a gas. Such a detector will permit the measurement of microdosimetric distributions in volumes that correspond to nanometers in a biological cell, e.g., the diameter of a DNA molecule. When a charged particle penetrates a gas, electrons produced during ionization events quickly become thermalized. The proposed detector would determine the positions of all of these electrons by imposing a suitable rf field, causing the electrons to agitate and excite molecules of the gas which would then fluoresce producing photons that could be detected by an external optical detector. In Phase I an experimental proof of concept would be made that a single charged particle track could be detected optically. In Phase II, vidicon or other CCD detectors would be used to obtain spatially resolved images of all of the electrons produced in the track of a single charged particle. In Phase III the new detector would be developed and marketed.